What is cystinuria?
Cystinuria is an inherited disease that causes stones made ofthe amino acid cystine to form in the kidneys, bladder, and ureters. Inherited diseases are passed down from parents to children through a defect in their genes. To get cystinuria, a person must inherit the defect from both parents.
The defect in the gene causes cystine to accumulate inside the kidneys, which are the organs that help regulate what goes in and out of your bloodstream. The kidneys have many functions, including:
- reabsorbing essential minerals and proteins back into the body
- filtering the blood to remove toxic waste
- producing urine to expel waste from the body
In someone who has cystinuria, the amino acid cystine builds up and forms stones instead of going back into the bloodstream. These stones can get stuck in the kidneys, bladder, and ureters. This can be very painful until the stones pass through urination. Very large stones may need to be surgically removed.
The stones can recur many times. Treatments are available to manage pain and to prevent more stones from forming.
Although cystinuria is a lifelong condition, symptoms typically first occur in young adults, according to a study in the European Journal of Urology. There have been rare cases in infants and adolescents. The symptoms may include:
- blood in the urine
- severe pain in the side or the back, almost always on one side
- nausea and vomiting
- pain near the groin, pelvis, or abdomen
Cystinuria is asymptomatic, meaning it causes no symptoms, when there are no stones. However, the symptoms will recur each time stones form in the kidneys. The stones commonly occur more than once.
Defects, also called mutations, in the genes SLC3A1 and SLC7A9 cause cystinuria. These genes provide the instructions for your body to make a certain transporter protein found in the kidneys. This protein normally controls the reabsorption of certain amino acids.
Amino acids are formed when the body digests and breaks down proteins. They’re used to perform a wide variety of bodily functions, so they’re important to your body and aren’t considered waste. Therefore, when these amino acids enter the kidneys, they’re normally absorbed back into the bloodstream. In people with cystinuria, the genetic defect interferes with the transporter protein’s ability to reabsorb the amino acids.
One of the amino acids — cystine — isn’t very soluble in urine. If it isn’t reabsorbed, it will accumulate inside the kidney and form crystals, or cystine stones. The rock-hard stones then get stuck in the kidneys, bladder, and ureters. This can be very painful.
You’re at risk of getting cystinuria only if your parents have the specific defect in their gene that causes the disease. As well, you only get the disease if you inherit the defect from both of your parents. Cystinuria occurs in about 1 in every 10,000 people around the world, so it’s fairly rare.
Cystinuria is usually diagnosed when someone experiences an episode of kidney stones. A diagnosis is then made by testing the stones to see if they are made out of cystine. Rarely is genetic testing done. Additional diagnostic testing could include the following:
24-hour urine collection
You will be asked to collect your urine in a container over the course of an entire day. The urine will then be sent to a laboratory for analysis.
An X-ray examination of the kidneys, bladder, and ureters, this method uses a dye in the bloodstream to help see the stones.
Abdominal CT scan
This type of CT scan uses X-rays to create images of the structures inside the abdomen to look for stones inside the kidneys.
This is an examination of urine in a laboratory that may involve looking at the color and physical appearance of the urine, viewing the urine under a microscope, and conducting chemical tests to detect certain substances, such as cystine.
If not treated properly, cystinuria can be extremely painful and may lead to serious complications. These complications include:
- kidney or bladder damage from a stone
- urinary tract infections
- kidney infections
- ureteral obstruction, a blockage of the ureter, the tube that drains urine from the kidneys into the bladder
How is cystinuria treated? | Treatment
Changes to your diet, medications, and surgery are options for treating the stones that form due to cystinuria.
Reducing salt intake to less than 2 grams per day has also been shown to be helpful in preventing stone formation, according to a study in the European Journal of Urology.
Adjusting pH balance
Cystine is more soluble in urine at a higher pH, which is a measure of how acidic or basic a substance is. Alkalinizing agents, such as potassium citrate or acetazolamide, will increase the pH of urine to make cystine more soluble. Some alkalinizing medications can be purchased over the counter. You should talk to your doctor before taking any type of supplement.
Medications known as chelating agents will help to dissolve cystine crystals. These drugs work by chemically combining with the cystine to form a complex that can then dissolve in urine. Examples include D-penicillamine and alpha-mercaptopropionylglycine. D-penicillamine is effective, but it has many side effects.
Pain medications may also be prescribed to control pain while the stones pass through the bladder and out of the body.
If the stones are very large and painful, or block one of the tubes leading from the kidney, they might need to be removed surgically. There are a few different types of surgeries to break up the stones. These include the following procedures:
- Extracorporeal shock wave lithotripsy (ESWL):This procedure uses shock waves to break up large stones into smaller pieces. It’s not as effective for cystine stones as for other types of kidney stones.
- Percutaneous nephrostolithotomy (or nephrolithotomy): This procedure involves passing a special instrument through your skin and into your kidney to take out the stones or break them apart.
Cystinuria is a lifelong condition that can be managed effectively with treatment. The stones appear most commonly in young adults under the age of 40 and may occur less frequently with age.
Cystinuria doesn’t affect any other parts of the body. The condition rarely results in kidney failure. Frequent stone formation causing blockage, and the surgical procedures that may be required as a result, can impact kidney function over time, according to the Rare Diseases Network.
Cystinuria can’t be prevented if both parents are carrying a copy of the genetic defect. However, drinking large amounts of water, reducing your salt intake, and taking medication can help to prevent stones from forming in the kidneys.